I have a 1972 C20 with a Eaton rear end with 4.57's....ok so I thought since it was geared so low and that it is a 3/4 ton it has to have posi...but its wierd...the other day me and another truck pulled againts each other (1999 F-150 I killed him) and when I was pulling him around I only had one tire spinning...but then today My tires spun in the gravel and I had both tires spinning....im really confused

Maybe under a small load(gravel) it works but on a heavy load the clutch material slips abit as in worn out clutch pack. Probable, you will just have to pull the cover and check. Maybe look to see if it has a posi and if it does try new 80/90 and add some posi lube also and it will pickup, the posilube does wear out. Worth a shot.

Simple way to find out. Jack up the rear end so both tires are off the ground. While in neutral, pick a tire and turn it. If the other tire spins in the opposite direction, you have an open (one legger) dif. If it spins in the same direction, its a posi.

Duh, limited slip additive does not make the clutchs slip easier not at all, it helps the clutch pack to lock up better, that is what it is for and it's only purpose. As for turning 1 tire if it is a govenor stye posi it needs pressure and speed to actually lockup, it won't turning it by hand only clutch only types do this. As stated by all the real only way is to pull the cover and at least you can inspect the oil, the gears and for posi when apart. Rip it apart dude, and if it is a posi and the oil looks good (no shiny flakes of clutch material) then use additive with the diff oil or it won't work properly. Good luck. By the way really nice truck, only see 1/2 tons nice to see a 3/4 ton.

Most people know that if you leave the additive out it will make the clutches hold better. For a drag race car you want to leave the additive out completely. For street cars you want the clutches to slip smooth when turning corners, and that's what the additive does. The less additive you use the more grip the clutches will have. Too much additive will allow the clutches to slip too easy and you will be spinning one wheel.

I've been in the rear end business a long time. I've also been an oil dealer for over 13 years. I've been around and I know what I'm talking about.

I have never seen a "clutch" style posi in a Eaton diff. If it has anything it will be a Detroit Locker. Adding additive will do no good. If it does have a detroit it may have some broken pieces causing it to act up.

What kind of additive are you talking about...are you talking about Differential oil?...and your saying the more oil in the differential the more it slips?.....O and I have been noticing when I let my clutch out my truck pulls a little and starts you jump a little until I let the clutch all the way out...but thats prolly just old clutch plates right?

All manufacturers have their own limited slip oil additive. This is a 4 oz bottle that is added to the gear oil. The jumping that you feel when letting out the clutch is probably clutch chatter and has nothing to do with the rear end.

Ok not saying I know it all, but thats what we were taught in automotive class year after year, it's in our school books. Could the SAE people be wrong.... maybe. Just saying what we were taught in school. Maybe I'm wrong. Some one find out for sure, or maybe bighead is.

I have never seen a "clutch" style posi in a Eaton diff. If it has anything it will be a Detroit Locker. Adding additive will do no good. If it does have a detroit it may have some broken pieces causing it to act up.

Thanks for the link but I have a full inventory of the Eaton Posi's so I'm well aware of them.

If you look at the OP's first post he stated he has a Eaton differential. The Eaton diff is a HO52 or a HO72 Differential. I've been dealing with the HO differentials for many many years and I've NEVER seen a clutch style posi used in a HO as they have all been Detroit Lockers. So therefore telling the OP to put posi additive in it is incorrect as it will do no good since the Detroit has NO cutches and is gear driven.

The OP can pull the rear cover on the diff and look in between the holes in the carrier and see if its a Detroit or if its just a open unit. And yes the HO differential is a "drop out" differential but it also has a removable rear cover.

So there ya go Pepi. You got your cherry popped on HO52/72 Differentials

Ok im not sure if I asked this on here or not but ill ask anyways...1972 C20 Eaton rearend ,4.57 gears, 4 speed manual, 250 ci motor...ok my truck should have posi right? cuz its a 3/4 ton and those really low gears....sometimes both spin but sometimes only one spins...and I dont wanna pull the cover cuz i just put oil in it....what do you think...o also there is no clips that I have to remove to take the axles out I take out the bolts and they slide right out....is that right?

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